Exam 03 - Version 059 – EXAM 3 – Fakhreddine –(53605...

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Unformatted text preview: Version 059 – EXAM 3 – Fakhreddine – (53605) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 12.0 points 100 mL of a 0.40 M solution of HClO is titrated with a solution of 1.6 M NaOH. What is the pH after the addition of 25.0 mL of the NaOH solution? The ionization constant for HClO is 3 . 5 × 10- 8 . The final volume of the solution is important. 1. 9.12 2. 10.5 correct 3. 9.87 4. 7.46 5. 7.00 6. 10.02 7. 8.92 8. 3.52 Explanation: [HA] = (0 . 4 mol / L)(100 mL) = 40 mmol [OH- ] = (1 . 6 mol / L)(25 mL) = 40 mmol V fin = 125 mL = 0.125 L K w = 10- 14 [ClO- ] = . 04 mol A- . 125 L = 0 . 32 m / L Analyzing the reaction using molarities, ClO- + H 2 O → HClO + OH- ini . 32 Δ- x x x fin . 32- x x x For ClO- , K b = K w K a = 1 × 10- 14 3 . 5 × 10- 8 = 2 . 85714 × 10- 7 x 2 . 32- x = 2 . 85714 × 10- 7 x 2 + 2 . 85714 × 10- 7 x- 9 . 14286 × 10- 8 = 0 Applying the quadratic formula, since b 2- 4 ac = (2 . 85714 × 10- 7 ) 2- 4(1)(- 9 . 14286 × 10- 8 ) = 3 . 65714 × 10- 7 , x =- 2 . 85714 × 10- 7 + √ 3 . 65714 × 10- 7 2 = 0 . 000302229 Thus [OH- ] = x = 0 . 000302229 , pOH =- log(0 . 000302229) = 3 . 51966 pH = 14- pOH = 10 . 4803 002 10.0 points The following solutions are mixed in equal volumes. I) 0.1 M HCl and 0.1 M NaCl II) 0.1 M HOCl and 0.1 M NaCl III) 0.1 M HOCl and 0.1 M NaOCl IV) 0.1 M HCl and 0.1 M NaOCl Which will give (a) buffer solution(s)? 1. III only correct 2. II and III only 3. IV only 4. II and IV only 5. I and II only Explanation: A buffer must contain a weak acid/base conjugate pair. OCl- is the conjugate base of the weak acid HOCl. HCl/Cl- is a stong acid conjugate pair. 003 10.0 points Methyl orange is an indicator with a K a of 10- 4 . Its acid form HIn is red, while its base form In- is yellow. At pH 6.0, the indicator Version 059 – EXAM 3 – Fakhreddine – (53605) 2 will be 1. red. 2. colorless. 3. yellow. correct 4. orange. 5. blue. Explanation: 004 10.0 points Mg(OH) 2 would be LEAST soluble in 1. 1 M NaOH. 2. 2 M HClO 4 . 3. pure water. 4. 1 M Ca(OH) 2 . correct 5. 1 M HNO 3 . Explanation: 005 12.0 points A solution is prepared by mixing 250 mL of 1.00 M CH 3 COOH with 500 mL of 1.00 M NaCH 3 COO. What is the pH of this solution?...
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This note was uploaded on 04/15/2009 for the course CH 53625 taught by Professor Fakhreddine during the Spring '09 term at University of Texas.

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Exam 03 - Version 059 – EXAM 3 – Fakhreddine –(53605...

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